2008 Jan 16 - through Meade ETX-125 telescope

The next image was through a Meade ETX-125 Maksutov-Cassegrain telescope. The camera was attached by a T-mount at the prime focus, so the telescope was acting as a 1900mm lens with 125mm aperture (so photographically f/15). The telescope was on an HEQ5 equatorial mount driven to follow the stars. The mounting arrangement can be seen here.

37 images have been combined by GRIP to make this photo. This time the nebula nearly fills the whole frame, whereas the non-telescopic example below was a 1:1 crop from the centre of the frame. So now I have obtained something which is printable at a decent size.

2008 Jan 10 - without a telescope

My photos since 19th December 07 have been taken with the camera mounted by itself on a HEQ5 motorised equatorial mount. This is definitely showing benefits.

The blueness of this photo (and most of mine) is because it was taken through suburban light pollution and therefore more background red has been removed than any other colour during GRIP's processing. It is always difficult to photograph the true colour of astronomical objects.

The points on the brightest star at the bottom (44 Ori, magnitude 2.75) are an artefact from the camera, thought to be due to diffraction from the iris blades in the lens. Stars visible in the photo go down to fainter than magnitude 14. That is, about 1/1500th of the brightness of the faintest stars visible to the naked eye.

Below is the same portion of the first frame of the 24 that were combined by GRIP to make the image above. It is very noisy because it was taken at ISO3200 and the sky is red from local street lights. Comparing this with the result above shows quite clearly that the automated astro-process in GRIP has a really useful effect.

The Trapezium, Theta Orionis

There is a group of young stars embedded in the Great Orion Nebula (M42) that are completely overexposed when photographing the fainter parts of the nebula. The 4 brightest members of the group are known as the Trapezium and also as the multiple star θ Orionis. The brightest of those has magnitude 5.1 and the faintest is variable between 8.0 and 8.5.

I wanted to find out what a correct exposure setting would be for them, through my 254mm f/4.8 Newtonian telescope. I took a sequence of different exposures first with the camera at prime focus and then with a 2x Barlow lens.

At prime focus 1/60s at ISO6400 was the longest exposure that just failed to saturate the brightest of the 4 stars, as shown by this line profile on a highly magnified portion of the image in GRIP:

The Lucky Imaging web site suggests that using video at 30 frames per second and then combining the best frames is a good way of overcoming atmospheric turbulence. So I might try it: my investigation above implies that ISO 3200 would be suitable at the prime focus of my telescope. It would also be interesting to see whether the technique would enable me to photograph the other fainter stars in the Trapezium area.

With the Barlow lens (2x linear magnification, so 4x areal) the corresponding image and profile was for 1/15s at ISO 6400:

(I am not happy with the colour fringing caused by the Barlow lens, considering the Trapezium was fairly central in the full image.)

In neither of the short exposure images above is there any trace of the nebula. So it is not possible to get the Trapezium correctly exposed and the nebula in the same frame.

At prime focus, 1s at ISO 6400 is the longest exposure before the 4 stars merge into a single blob and also shows the central part of the nebula:

The sequence on the right shows successive halvings of exposure time at ISO 6400, from 15s at the top to 1/60s at the bottom (all at prime focus: 1200mm f/4.8). The bottom one is the only one in which none of the 4 Trapezium stars is saturated.